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 // SPDX-License-Identifier: LGPL-3.0-or-later
 // Copyright (C) 2022 - 2025 Whitney Armstrong, Wouter Deconinck, Sylvester Joosten, Dmitry Romanov, Yann Bedfer
 
 /*
   Digitization specific to MPGDs.
   - What's special in MPGDs is their 2D-strip readout: i.e. simultaneous
    registering along two sets of coordinate strips.
   - The "process" method involves a combination of segmentation, simulation and
    digitization.
   - The segmentation done when producing "sim_hits", and stored as a pixel
    cellID, is overwritten by strip cellIDs, via "dd4hep::MultiSegmentation".
   - The simulation will eventually involve simulating the amplitude and timing
    correlation of the two coordinates, and the spreading of the charge on
    adjacent strips producing multi-hit clusters.
     The preliminary version is rudimentary: single-hit clusters, with identical
    timing and uncorrelated amplitudes.
   - The digitization follows the standard steps, only that it needs to be
    convoluted with the simulation.
   NOTA BENE: The file could be simplified, see issue #1702.
  */
 
 #include "MPGDTrackerDigi.h"
 
 #include <DD4hep/Alignments.h>
 #include <DD4hep/DetElement.h>
 #include <DD4hep/Handle.h>
 #include <DD4hep/IDDescriptor.h>
 #include <DD4hep/Objects.h>
 #include <DD4hep/Readout.h>
 #include <DD4hep/VolumeManager.h>
 #include <DD4hep/detail/SegmentationsInterna.h>
 #include <DDSegmentation/BitFieldCoder.h>
 #include <Evaluator/DD4hepUnits.h>
 #include <JANA/JException.h>
 #include <Math/GenVector/Cartesian3D.h>
 #include <Math/GenVector/DisplacementVector3D.h>
 #include <Parsers/Primitives.h>
 // Access "algorithms:GeoSvc"
 #include <algorithms/geo.h>
 #include <algorithms/logger.h>
 #include <edm4hep/MCParticleCollection.h>
 #include <edm4hep/Vector3d.h>
 #include <edm4hep/Vector3f.h>
 #include <fmt/core.h>
 #include <algorithm>
 #include <cmath>
 #include <cstdint>
 #include <gsl/pointers>
 #include <initializer_list>
 #include <iterator>
 #include <random>
 #include <unordered_map>
 #include <utility>
 #include <vector>
 
 #include "algorithms/digi/MPGDTrackerDigiConfig.h"
 
 using namespace dd4hep;
 
 namespace eicrecon {
 
 void MPGDTrackerDigi::init() {
   // Access id decoder
   m_detector                            = algorithms::GeoSvc::instance().detector();
   const dd4hep::BitFieldCoder* m_id_dec = nullptr;
   if (m_cfg.readout.empty()) {
     throw JException("Readout is empty");
   }
   try {
     m_seg    = m_detector->readout(m_cfg.readout).segmentation();
     m_id_dec = m_detector->readout(m_cfg.readout).idSpec().decoder();
   } catch (...) {
     critical("Failed to load ID decoder for \"{}\" readout.", m_cfg.readout);
     throw JException("Failed to load ID decoder");
   }
   // Method "process" relies on a strict assumption on the IDDescriptor:
   // - Must have a "strip" field.
   // - That "strip" field includes bits 30|31.
   // Let's check.
   if (m_id_dec->get(((CellID)0x3) << 30, "strip") != 0x3) {
     critical(R"(Missing or invalid "strip" field in IDDescriptor for "{}"
         readout.)",
              m_cfg.readout);
     throw JException("Invalid IDDescriptor");
   }
   debug(R"(Find valid "strip" field in IDDescriptor for "{}" readout.)", m_cfg.readout);
 }
 
 void MPGDTrackerDigi::process(const MPGDTrackerDigi::Input& input,
                               const MPGDTrackerDigi::Output& output) const {
 
   // ********** SIMULATE THE 2D-strip READOUT of MPGDs.
   // - Overwrite and extend segmentation stored in "sim_hit", which is anyway
   //  expected to be along a single coordinate (this happens to allow one to
   //  reconstruct data w/ a segmentation differing from that used when
   //  generating the data).
   // - New segmentation is along two coordinates, described by two cellID's
   //  with each a distinctive "strip" field.
   //   N.B.: Assumptions on the IDDescriptor: the "strip" specification
   //  is fixed = cellID>>32&0x3.
   // - The simulation is simplistic: single-hit cluster per coordinate.
 
   const auto [headers, sim_hits] = input;
   auto [raw_hits, associations]  = output;
 
   // local random generator
   auto seed = m_uid.getUniqueID(*headers, name());
   std::default_random_engine generator(seed);
   std::normal_distribution<double> gaussian;
 
   // A map of unique cellIDs with temporary structure RawHit
   std::unordered_map<std::uint64_t, edm4eic::MutableRawTrackerHit> cell_hit_map;
   // Prepare for strip segmentation
   const Position dummy(0, 0, 0);
   const VolumeManager& volman = m_detector->volumeManager();
 
   using CellIDs = std::pair<CellID, CellID>;
   using Sim2IDs = std::vector<CellIDs>;
   Sim2IDs sim2IDs;
   for (const edm4hep::SimTrackerHit& sim_hit : *sim_hits) {
 
     // ***** TIME SMEARING
     // - Simplistic treatment.
     // - A more realistic one would have to distinguish a smearing common to
     //  both coordinates of the 2D-strip readout (due to the drifting of the
     //  leading primary electrons) from other smearing effects, specific to
     //  each coordinate.
     double time_smearing = gaussian(generator) * m_cfg.timeResolution;
     double result_time   = sim_hit.getTime() + time_smearing;
     auto hit_time_stamp  = (std::int32_t)(result_time * 1e3);
 
     // ***** SEGMENTATION
     // - The two cellID's are encoded via a "dd4hep::MultiSegmentation"
     //  discriminating on the strip field, w/ "strip" setting of 0x1 (
     //  called 'p') and 0x2 (called 'n').
     // - They are evaluated based on "sim_hit" Cartesian coordinates
     //  positions
     //   Given that all the segmentation classes foreseen for MPGDs (
     //  "CartesianGrid.." for Outer and EndCaps, "CylindricalGridPhiZ" for
     //  "CyMBaL") disregard the _global_ position argument to
     //  "dd4hep::Segmentation::cellID", we need the _local_ position and
     //  only that.
     const edm4hep::Vector3d& pos = sim_hit.getPosition();
     using dd4hep::mm;
     Position gpos(pos.x * mm, pos.y * mm, pos.z * mm);
     CellID vID = // Note: Only the bits corresponding to the volumeID will
         // be used. The rest, encoding the segmentation stored in "sim_hit",
         // being disregared.
         sim_hit.getCellID();
     DetElement local = volman.lookupDetElement(vID);
     const auto lpos  = local.nominal().worldToLocal(gpos);
     // p "strip"
     CellID stripBitp = ((CellID)0x1) << 30;
     CellID vIDp      = vID | stripBitp;
     CellID cIDp      = m_seg->cellID(lpos, dummy, vIDp);
     // n "strip"
     CellID stripBitn = ((CellID)0x2) << 30;
     CellID vIDn      = vID | stripBitn;
     CellID cIDn      = m_seg->cellID(lpos, dummy, vIDn);
 
     sim2IDs.emplace_back(cIDp, cIDn); // Remember cellIDs.
     // ***** DEBUGGING INFO
     if (level() >= algorithms::LogLevel::kDebug) {
       CellID hIDp = cIDp >> 32;
       CellID sIDp = cIDp >> 30 & 0x3;
       debug("--------------------");
       debug("Hit cellIDp  = 0x{:08x}, 0x{:08x} 0x{:02x}", hIDp, vIDp, sIDp);
       CellID hIDn = cIDn >> 32;
       CellID sIDn = cIDn >> 30 & 0x3;
       debug("Hit cellIDn  = 0x{:08x}, 0x{:08x} 0x{:02x}", hIDn, vIDn, sIDn);
       debug("   position  = ({:.2f}, {:.2f}, {:.2f})", sim_hit.getPosition().x,
             sim_hit.getPosition().y, sim_hit.getPosition().z);
       debug("   xy_radius = {:.2f}", std::hypot(sim_hit.getPosition().x, sim_hit.getPosition().y));
       debug("   momentum  = ({:.2f}, {:.2f}, {:.2f})", sim_hit.getMomentum().x,
             sim_hit.getMomentum().y, sim_hit.getMomentum().z);
       debug("   edep = {:.2f}", sim_hit.getEDep());
       debug("   time = {:.4f}[ns]", sim_hit.getTime());
       debug("   particle time = {}[ns]", sim_hit.getParticle().getTime());
       debug("   time smearing: {:.4f}, resulting time = {:.4f} [ns]", time_smearing, result_time);
       debug("   hit_time_stamp: {} [~ps]", hit_time_stamp);
     }
 
     // ***** APPLY THRESHOLD
     if (sim_hit.getEDep() < m_cfg.threshold) {
       debug("  edep is below threshold of {:.2f} [keV]", m_cfg.threshold / keV);
       continue;
     }
 
     // ***** HIT ACCUMULATION
     for (CellID cID : {cIDp, cIDn}) {
       if (!cell_hit_map.contains(cID)) {
         // This cell doesn't have hits
         cell_hit_map[cID] = {
             cID, (std::int32_t)std::llround(sim_hit.getEDep() * 1e6),
             hit_time_stamp // ns->ps
         };
       } else {
         // There is previous values in the cell
         auto& hit = cell_hit_map[cID];
         debug("  Hit already exists in cell ID={}, prev. hit time: {}", cID, hit.getTimeStamp());
 
         // keep earliest time for hit
         hit.setTimeStamp(std::min(hit_time_stamp, hit.getTimeStamp()));
 
         // sum deposited energy
         auto charge = hit.getCharge();
         hit.setCharge(charge + (std::int32_t)std::llround(sim_hit.getEDep() * 1e6));
       }
     }
   }
 
   // ***** raw_hit INSTANTIATION AND raw<-sim_hit's ASSOCIATION
   for (auto item : cell_hit_map) {
     raw_hits->push_back(item.second);
     auto sim_it = sim2IDs.cbegin();
     for (const auto& sim_hit : *sim_hits) {
       CellIDs cIDs = *sim_it++;
       for (CellID cID : {cIDs.first, cIDs.second}) {
         if (item.first == cID) {
           // set association
           auto hitassoc = associations->create();
           hitassoc.setWeight(1.0);
           hitassoc.setRawHit(item.second);
           hitassoc.setSimHit(sim_hit);
         }
       }
     }
   }
 }
 
 } // namespace eicrecon

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